In ink-jet printing tiny drops of ink fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the ink drops image-wise onto the ink-receiver.
In most cases, printing is accomplished by moving the print head in a direction across the ink receiver (the main scan direction), while at the same time the ink receiver is advanced in a direction perpendicular to the main scan direction (the sub scan direction). The print head of an inkjet colour printer generally comprises a plurality of head-segments arrayed in the main scan direction. Each head segment has a plurality of nozzles arranged at different locations in the sub scan direction. Colour printing is performed in accordance with the subtractive colour model. The subtractive colour model is represented typically by a combination CMY, of cyan (C), magenta (M) and yellow (Y) inks or a more typical combination, CMYK, of CMY plus black (K) ink. A typical configuration of a print head is four head-segments, one per colour, arranged in a nozzle order of KCMY so that when printing in a unidirectional mode the K ink is the first to be jetted on the ink receiver, followed by C, M, and finally Y ink. However, to reduce print time the printers are generally designed with a bi-directional mode of printing capability.
In bi-directional mode printing, the print head with a nozzle order KCMY would print a patch with an ink order of K, C, M, and Y from left-to-right and a print patch with a reversed ink order of Y, M, C, and K when printing from right-to-left. This results in a noticeable chromatic variation in adjacent print patches. A patch printed with an ink order of K, C, M, and Y appears “lighter” to the human eye than a patch printed with an ink order of Y, M, C, and K. This phenomenon is due to the fact that each of the four standard subtractive process colours has a unique brightness distinguishable by the human eye. As the black ink K absorbs most of the light it can be regarded as the most critical ink for chromatic variations.
This image quality defect, usually referred to as “bi-directional hue shift”, is reduced in EP 737001 A (XEROX) by introducing a complex colour management system having two colour mapping look-up tables. The first colour mapping look-up table is used when the print head travels from left-to-right, while the second colour mapping look-up table is used when the print head travels back from right-to-left. Similar methods for colour correction in bi-directional printing are disclosed in EP 1048475 A (HEWLETT-PACKARD) and US 20030048327 (HEWLETT-PACKARD).
Another approach is disclosed by EP 1208991 A (OCE TECH) which doubles the number of head-segments of the print head by having two head-segments for each colour and arranging them symmetrically in the main scan direction. This results in a print head having the following sequence of head-segments: C1, M1, Y1, Y2, M2, and C2. In printing from left-to-right the head-segments C1, M1, and Y1 are used, while in printing from right-to-left the head-segments Y2, M2, and C2 are used. Hereby the cyan ink is always the first to be jetted on the ink receiver, followed by the magenta ink, and finally the yellow ink. Similar printing methods for reducing bi-directional hue shift by using print head with a plurality of head-segments are disclosed in EP 1273453 A (HEWLETT-PACKARD) and U.S. Pat. No. 6,273,550 (MUTOH).
Ink compositions for ink-jet typically include following ingredients: dyes or pigments, water and/or organic solvents, humectants such as glycols, detergents, thickeners, polymeric binders, preservatives, etc. It will be readily understood that the optimal composition of such ink is dependent on the ink-jetting method used and on the nature of the ink-receiver to be printed. The ink compositions can be roughly divided in:                water-based, the drying mechanism involving absorption, penetration and evaporation;        oil-based, the drying involving absorption and penetration;        solvent-based, the drying primarily involving evaporation;        hot melt or phase change, in which the ink is liquid at the ejection temperature but solid at room temperature and wherein drying is replaced by solidification;        UV-curable, in which drying is replaced by polymerization.        
Water-based inks suffer from a number of disadvantages such as: (a) their requiring water evaporation and therefore an extensive drying system, especially when printing speed is important; (b) the tendency of large printed areas to cockle, (c) sensitivity of images to wet and dry rubbing, (d) the tendency of low viscosity inks to dry at the tip of the orifice. The use of polar solvent-based inks can overcome some of the problems inherent in water-based inks, but results in other problems such as the possible generation of toxic or inflammable vapours. Therefore efforts were made to develop low-solvent ink compositions from which the concept of UV-curable ink compositions emerged.
It will be readily understood that the first two types of ink compositions are more suitable for a receiving medium that is more or less absorptive, whereas UV-curable inks are more suitable for non-absorbent ink-receivers.
However, the behaviour and interaction of a UV-curable ink on a substantially non-absorbing ink-receiver was found to be quite complicated compared to a water- or a solvent-based ink on absorbent ink-receivers. For example, a good and controlled spreading of the ink on the substrate is problematic. Also droplets of different colour inks are jetted on top of one another, causing problems such as coalescence of adjacent ink dots. To reduce these problems the curing means can be arranged in combination with the print head of the ink-jet printer, travelling therewith so that images printed upon the surface of the ink-receiver are exposed to curing radiation very shortly after having been printed upon the ink-receiver. In US 2004006157, the jetting and curing of the ink is performed by an ink jet printhead with an ultra violet lamp attached to the printhead. This further enlarges the print head and complicates its design.
Accordingly, it would be highly desirable to have a UV-curable ink-jet printing process that reduces bi-directional hue shift without compromising throughput and without requiring extra cost or complexity in the printing process.